1. Technical Field
This invention relates to compression molding equipment and, more particularly, to vacuum molds designed to draw a vacuum during the molding process.
2. Discussion
The use of vacuum during compression molding of parts has become more and more prevalent in the industry. The various advantages of using this technique are documented in the patent literature such as in commonly assigned U.S. Pat. Nos. 4,488,862; 4,551,085; 4,867,924; 4,612,149; 5,130,071 and 4,855,097. Still other mold designs and their purported advantages are disclosed in U.S. Pat. No. 3,840,239 to Fazekas et al, U.S. Pat. No. 4,597,929 to Blayne and in the article entitled "Surface Porosity and Smoothness of SMC Molding As Affected by Vacuum and Other Molding Variables", by J. D. Gorsuch, 33 Annual Technical Conference, 1978, Reinforced Plastic/Composites Institute, the Society of Plastic Industry, Inc.
While the designs in the commonly assigned U.S. patents have, for the most part, performed quite satisfactorily in production, they are, unfortunately, relatively expensive to make. It is also often impractical to refit these molds with new vacuum seals when it is desired to make a prototype part in which only a limited number of parts are to be made.
Another vacuum seal concept employs a vacuum seal arrangement in which a plate is connected to an upper die which engages a resilient seal on a lower die. This mold construction suffers from various disadvantages such as added cost required to grind heavy plates used as seal surfaces. Such mold constructions also require machine shop fitting to coordinate a seal between the upper and lower dies. The resilient seal mounted on the lower die encounters additional wear due to mold flash sticking or adhering to the resilient seal. The resilient seal on the lower die also has sharp corners which must fit tightly with the corresponding sharp corners of the seal plate on the upper die and consequently the resilient seal wears quickly.
Therefore, there exists a need for a lower cost, yet reliable, mold for use in molding prototype parts or in other situations where a limited number of parts are to be made.